The term "tetrahydrofuran-insoluble gel fraction" used herein means the extraction residue that results from 22-hour extraction with hot tetrahydrofuran by Soxhlet's extractor with a 350-mesh (British Standard No. 410-1962) filter, and the term "tetrahydrofuran-soluble fraction" means the portion extracted by said method.
It is known that a molding composition having a soft, rubbery touch, or so-called soft vinyl chloride resin composition, can be obtained by adding a plasticizer to a vinyl chloride resin. However, such soft vinyl chloride resins are generally poor in creep characteristics and inferior to rubber in recovery from stress, so that their applications to uses where a small compression permanent set is required such as packing are limited. For example, ordinary vulcanized rubbers have a compression permanent set (measured according to JIS K-6301 under the conditions of 70.degree. C., 22 hrs. and 25% compression) as small as about 20 to 45% and have good creep characteristics, whereas general soft vinyl chloride resins have a compression permanent set as large as about 65 to 80%, and are poor in creep characteristics.
Heretofore, vinyl chloride resins having a high average polymerization degree, such as, for example, 3,000 or more, have been used for producing a molded article with a small compression permanent set. However, the molded article obtained therefrom usually has a compression permanent set as high as about 60% which is far from the level allowable in actual uses. Such moldings also involve many other problems in working, such as necessity of high-temperature melting for molding and requirement for heat stability and strong extrusion force.
General utility vinyl chloride resins normally contain no tetrahydrofuran-insoluble gel fraction, and if such a gel fraction exists in the vinyl chloride resin, many fish eyes are formed in the molded article obtained therefrom and also its surface smoothness is worsened because of poor processability, so that it is considered in the art that the presence of any gel fraction in vinyl chloride resin renders the product commercially valueless.
A method is also known for polymerizing vinyl chloride in the presence of a polyfunctional monomer having two or more ethylenic double bonds in the molecule to produce a vinyl chloride resin with an average polymerization degree (measured according to JIS K-6721) of 1,300 to 4,000 which is generally referred to as high polymerization grade. According to this method, however, a small quantity of gel fraction is often produced in the vinyl chloride resin depending on the type of the polyfunctional monomer used, the polymerization conditions and other factors, but since the object is definitely to obtain a vinyl chloride resin with a high polymerization degree, an effort has been made to not form such a gel fraction.